Electrostatically-assisted two-step conductive polymer applique (CPA) paint removal process

ABSTRACT

A molecular adhesive system for reversibly joining two surfaces, comprising: an anionic coating on a first of two surfaces to be joined; a conductive polymer nanotube array on a second of the two surfaces to be joined; wherein said conductive polymer nanotube array is functionalized with metal nanoparticles; and an electric potential applied across said two surfaces.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein may be manufactured and used by or forthe government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

Coatings used on ships, combat vehicles, and structures perform avariety of functions including protection from moisture, heat, saltspray and other chemicals. For the Air Force and Marines, fresh coatingsare applied to change camouflage patterns or to change insignias.Additionally, new coatings are applied to spruce up the aircraft or shipbefore an inspection or an open house. These coatings must be removedfrom the metal in sections, to allow detail work to the metal surfacerework or repair operations, and to keep weight down to acceptablelevels. Over the past fifteen years, numerous efforts have beeninvestigated to reduce the hazardous waste generated by various paintremoval systems. Highly toxic materials such as phenols, methylenechloride, benzyl alcohol, chemical immersion, plastic media blasting andlasers have not proven to be robust enough nor environmentally compliantfor paint removal on ships, structures, and military vehicles. In orderto comply with existing environmental regulations and to effectivelyremove paint formulations from various substrates, novel approaches topaint stripping are being investigated.

Over the past decade, there has been considerable effort in producingzero-VOC coatings for use on exterior aircraft surfaces. Appliques aremanufactured films of fluoropolymer with a pressure-sensitive adhesiveto bond to aircraft surfaces. Such materials have been developed by 3MCorporation and units of the BOEING Company to replace paint with alaminate film (applique) that is easily applied and removed(peel-and-stick), waterproof and weighs less than accumulated coats ofpaint.

Looking to nature for safer adhesives, geckos in particular, havedeveloped the most complex adhesion system in their foot pads. Grayish,N., Wilkinson, M., and Autumn, K., Frictional and elastic energy ingecko adhesive detachment, J. R. Soc. Interfac, 5, 339 (2008). They havean adhesion architecture which allows them the ability to adhere anddetach from different smooth and rough surfaces at will. The peeling-offmechanism (or detachment from the surface) of the gecko requiresreorientation of the spatulae. This hierarchical structure gives thegecko the unique ability to create large real area contact with roughsurfaces. This adhesion action is termed “smart adhesion”. Weak van der.Waals (primary adhesion forces) and capillary forces (secondary forces)are the forces needed to adhere to various surfaces by the gecko.Autumn, K., Sitti, M., Liang, Y. A., Peattie, A. M., Hansen, W. R.,Sponberg, S., Kenny, T. W., Fearing, R., Israelachvili, N. J. and Full.Evidence for van der Waals adhesion in gecko setae, R. J., Proc. Natl.Acad. Sci., USA, 99, 12252 (2002).

In order to realize the potential for a simple, non-VOC paint removalsystem, the adhesion/mechanical properties of a synthetic gecko's footwould have to be modified. The finely structured contact elements thatwould be required to deliver sufficient adhesive force to remove a paintformulation from a metal substrate would be significant.

The current invention provides sufficient adhesion forces, utilizingelectrostatically enhanced, biomimetic adhesion of two surfaces usingnanostructures inspired by geckos' setae. The new materials do not onlyutilize van der Waals forces for their adhesive properties; they alsotake advantage of the chemical interaction of anions on a first surfacewith metal nanoparticles on a second surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the chemical formula of the preparation ofPAA-co-TEVS polymers, according to embodiments of the invention.

FIG. 2 is an illustration of the process of coating a surface withPAA-co-TEVS polymers, according to embodiments of the invention.

FIG. 3 is an illustration of the synthesis of conductive polymernanotube arrays with Fe⁰ nanoparticles, according to embodiments of theinvention.

FIG. 4 a is an illustration of the process of applying a nanotube arrayto a PAA layer, according to embodiments of the invention.

FIG. 4 b is an illustration of the process of applying adhesion forcesto peel off a PAA paint layer from a metal substrate, according toembodiments of the invention.

FIG. 4 c is an illustration of the process of reversing potential torelease a PAA-paint layer from a CPA layer, according to embodiments ofthe invention.

It is to be understood that the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not to be viewed as being restrictive of the invention, as claimed.Further advantages of this invention will be apparent after a review ofthe following detailed description of the disclosed embodiments, whichare illustrated schematically in the accompanying drawings and in theappended claims.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention generally relate to thesynthesis, preparation, and use of a low cost, environmentally benign,electrostatically enhanced biomimetic adhesive system. This system isuseful for protective coatings, removable protective coatings and in anenvironmentally safe paint and applique removal process.

The present invention addresses the limitations of current paintstrippers with a more robust technology; eliminating media and hazardouswaste by-products. A combination of nature's adhesive properties (e.g.gecko) with the adhesion properties of appliques provides a low cost,environmentally benign adhesive system useful, for example, as a paintremoval system.

Benefits of this present invention include, for example: 1) providingzero-VOC and zero-hazardous air pollutants (HAPs) paint and appliqueremoval, 2) enabling multiple coating removals from a variety of metalsubstrates, and 3) delivering an easily applied, cost-effective, andre-useable system for paint and applique removal.

Although embodiments of the invention are described in considerabledetail, including references to certain versions thereof, other versionsare possible. Therefore, the spirit and scope of the appended claimsshould not be limited to the description of versions included herein.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly disclosed. Each smaller range between any statedvalue or intervening value in a stated range and any other stated orintervening value in that stated range is encompassed within theinvention. The upper and lower limits of these smaller ranges mayindependently be included or excluded in the range, and each range whereeither, neither or both limits are included in the smaller ranges isalso encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

In one embodiment of the adhesive system of the present invention, thesystem is used as an adhesive to bond two surfaces. The system comprisesa first film or coating which provides a plurality of anionic moietieson a first surface to be bonded. A second film or coating on a secondsurface to be bonded provides a plurality of metal nanoparticles on thesecond surface. In examples, the surface to be bonded could be the filmor coating itself. The plurality of both the anionic moieties and themetal nanoparticles is a relative number, sufficient to provide theadhesion required to bond said 2 surfaces together with the desiredstrength. In preferred examples, the plurality of anionic moieties andthe plurality of metal nanoparticles necessary for the surfaces to bebonded requires complete coverage of both surfaces, more preferablycomplete charge coverage. The area of the surface is preferably coveredwith the same amount, for example, mole %, grams, charge density,density of mass of the anionic moieties and metal nanoparticles.

When the 2 surfaces are placed into contact, the anionic moieties andthe metal nanoparticles form ionic bonds, resulting in adhering the twosurfaces together. A further enhanced bond is formed when electricalpotential applied across the two adhering coatings/surfaces alters theoxidation state of the metal nanoparticles, facilitating the desiredionic bonding; and, when the opposite potential is applied, allowing forthe easy separation of the two surfaces.

The first surface to be bound has on its surface a plurality of anions.These anions may be inherent to the surface to be bonded or may requirethe application of a pretreatment film. Examples of such anions include,but are not limited to, CO₂ ⁻, OH⁻, tetrafluoroborate,hexafluorophosphate, trifluoromethanesulfonate, perchlorate, sulfate,chloride, bromide, and nitrate. These anions can be present in the formof, for example, tetrafluoroborate, hexafluorophosphate,trifluoromethanesulfonate, perchlorate, sulfate, chloride, bromide,nitrate, or functionalized polymers including, but not limited to,polyesters, polyethers, polyvinyl, polyalcohols, poly amides,polyoelfins, polyaromatics, polyacids, polysilanes, polysiloxanes,polyacrylic acid, polystyrene sulfonic acid, polymethacrylic acid, polyphosphonic acids, and the like.

In examples, incorporating a first surface comprising painted metal,preferred polyanionic coatings bind to the painted surface with enoughstrength to be stronger than the bond of the paint to the metal surfaceunderneath. The adhesion of paints onto a metal surface is the summationof all interfacial and intermolecular forces. These forces includechemical, mechanical, electrostatic and acid-base adhesion phenomena andtheir combinations. Chemisorption (covalent and co-ordination bonds)interactions are the predominant mechanism by which paint/polymersystems adhere to metals. Many other types of adhesive forces such ashydrogen bonding, van der Waals, electrostatic are also contributing tothe paint-metal adhesion.

One preferred example of bonding of the two surfaces incorporates thepreparation of a PAA (polyanionic applique) pretreatment polyanioniccoating on a first surface and a layer of CPA (conductive polymerapplique) nanotube arrays on a second surface.

In order to overcome the strong intra- and inter-molecular attractionsbetween the paint and (metal) substrate, a stronger bond between theConductive Polymer Applique (CPA) and the paint must be realized. Ionicor electrostatic bonds have the highest bond energies as compared toother types. The following table (see Table 1) summarizes the forces atthe interface or within the bulk of a material.

TABLE 1 Typical forces at the interface or within the bulk of a materialSource of Bond energy Type of Force Force (KJ/mol) Description PrimaryCovalent  60-700 Diamond or cross-linked polymers Primary Ionic or 600-1000 Crystals Electrostatic Primary Metallic 100-350 Forces inwelded joints Secondary Dispersion 0.1-40  Temporary dipoles (van derWaals) Secondary Polar  4-20 Permanent dipoles (van der Waals) SecondaryHydrogen Up to 40 Sharing of protons between (van der Waals) Bonding twoatoms possessing loan pairs of electrons

A coating layer comprising conductive polymer (CP) nanotube arrays isprovided on the second surface. This second surface can be used, forexample, as a pulling layer to remove paint/appliques from the firstsurface. Conductive polymer nanotubes, preferably based on anilinemonomers, pyrrole monomers, thiophene monomers, or combinations thereofcan be utilized due to their unique properties including ease offabrication, synthetic tenability, flexibility and high mechanicalproperties including high strength and the ability survive highmechanical strains and stresses. The CP nanotube arrays arefunctionalized with anionic groups (e.g. carboxylic, sulfonic, andphosphonic groups) for subsequent ionic attachment of metalnanoparticles, preferably ferrous metals. Preferably, a template-freemethod is used to fabricate the CP nanotube arrays.

Polyanionic Coating

Example polyanionic coatings include single layer coatings or multiplelayer coatings. Embodiments of multiple layer coatings may be comprisedof a first layer for binding to the paint, bound to a second layercontaining anions.

In one embodiment, a pretreatment polyanionic coating (PAA-co-TEVS) isprepared via co-polymerization of acrylic acid, polystyrene sulfonicacid, polymethacrylic acid, polystyrene phosphonic acid, for example,with triethyoxyvinylsilane (TEVS) (as an adhesion promoter) usingthermal initiators (e.g. 4,4′-azobis(4-cyanovaleric acid, ACVA), AIBN,benzoyl peroxide, and the like. See FIG. 1 , where n and m are eachindependently up to about 10,000.

The PAA-co-TEVS is coated onto the first surface to be bonded via, forexample, spraying, dipping, brushing, doctor blading, or the like. Afterit is deposited in its neutral form the PAA-co-TEVS will have anextended conformation with COOH groups on the surface of the filmavailable for subsequent hydrolysis to the CO2- salt; which will allowfor either ionic or electrostatic bonding for the CPA and the secondsurface. This PAA-co-TEVS layer is then hydrolyzed via treatment withaqueous acid or base, preferably an aqueous solution of any acid or basethat is about 0.1 molar. Examples include, dilute acid solutions ofhydrochloric acid, sulfuric acid, nitric acid, acetic acid (0.1-5.0Msolutions only) and sodium hydroxide, potassium hydroxide (0.1-5.0M)solutions only, these aqueous acid or base solutions must be lowmolarity. The hydrolyzed PAA film will serve as the anionic pretreatmentlayer for later attachment to the second surface—with the CPA nanotubesarray.

Conductive Polymer Applique (CPA)

As illustrated in FIG. 3 , preferred CP nanotube arrays are fabricatedby polymerizing the monomers on a substrate. Suitable substratesinclude, but are not limited to, metal alloys such as aluminum alloys:2024-T3, 7075-T6, 6061, 5083; ferrous alloys such as carbon steels:1008, 1010; high strength steels: 4130, 4340; and cold rolled steel. Inone example embodiment, a 3-thiophenepropionic acid monomer is electropolymerized on an Anodic Aluminum Oxide (AAO) membrane using boronfluoride-ethyl ether (BFEE) as a solvent. The CP nanotube polymer isthen purified to remove any carbon-based materials that don't have thedesired properties, for example, carbonaceous/graphitic impurities orimpurities that don't have the same shape as the nanotubes.

A preferred purification process comprises stirring the CP nanotubepolymer for long, slow oxidation in an H₂O₇ solution assisted byultrasonication at room temperature combined with a short acidtreatment. Purified CP nanotube polymers were obtained by stirring in 30wt. % H₂O₂ solution at room temperature for 7 days followed by HCltreatment.

After purification of the CP nanotube polymer, attachment of metalnanoparticles to the CP nanotube arrays is accomplished, for example,via a liquid-phase reaction utilizing sodium borohydride (NaBH₄) as thereducing agent.

Of the preferred ferrous metals, one preferred metal nanoparticle isiron, Fe⁰. The preparation of the Fe⁰ nanoparticles under ambientatmosphere will result in a thin film of iron oxide (˜1-2 nm) on thesurface of the nanoparticles. This thin iron oxide layer will providesites for ionic bond formation between the iron oxide surface and theanionic groups (e.g., CO₂ ⁻) on the CPA nanotube arrays. A preferredprocedure for preparing the Fe⁰ nanoparticles (10-30 nm) involvesagitation of FeSO₄.H₂O and the CP nanotube arrays in deionized water.This is followed by adjusting the pH to be slightly basic with sodiumhydroxide and reducing the Fe²⁺ and Fe³⁺ salts on the CP nanotube arraysto Fe⁰ using a reducing agent, for example, sodium borohydride (NaBH₄).NaBH₄ is a very mild reducing agent and will not reduce the carbonylfunctional groups on the CP nanotube arrays. An illustration of thisprocess is shown in FIG. 3 .

Paint Removal

Referencing FIGS. 2 and 4 a-4 e, when used as a paint removal system,the CPA nanotube arrays film will be placed in direct contact with thePAA pretreated paint layer. For additional adhesion of the CPA to thePAA pretreatment an electrical potential is applied across the films tooxidize the Fe⁰ nanoparticles to Fe⁺² with concurrent reduction of theAAO membrane to maintain charge balance. In one example, the structuralsurface under the paint functions as a first electrode, the otherelectrode would be separately applied to the layer with the ironnanoparticles. The oxidized Fe⁺² ions will adhere very strongly throughionic bond formation to the PAA pretreatment film (via the anionicgroups). Once the ionic bond is formed, removal of the coating via shearadhesive forces is required to peel-off the CPA/PAA-adhered paint. Whenthe potential is reversed, the Fe⁺² nanoparticles on the CPA film willbe reduced to Fe⁰ allowing for the CPA/PAA-adhered paint to fall-off theCPA applique which can then be used again to remove more (PAA treated)paint—which represents a unique paint removal process which isnon-toxic, reversible, and reusable.

EXAMPLES Example 1

Electrolytically assisted paint removal from a metal substrate has beenshown to effectively de-bond paint from a metal surface. A metalsubstrate was made cathodic in an electrolyte solution (pH=6-8) and ananode was added to complete the cell. Electric current was applied tothe electrolytic cell (40-140 mA/cm2) for 5-60 minutes. De-bonding ofthe paint was demonstrated in plating tanks that can hold electrolytesolutions.

Example 2

A similar approach that is more robust which can remove paint from avariety of substrates and shapes can be achieved with the currentinvention. A polyanionic pretreatment (PAA) is directly attached to thepaint on a first surface. Fabrication of the CPA nanotube arrays is viaa hard template. The hard template used for the fabrication process is aporous anodic aluminum oxide (AAO) membrane. The CPA nanotube arrayscontaining. Fe⁰ nanoparticles can undergo redox chemistry which assuresthe formation of an ionic bond between the two layers. The formation ofthis ionic bond is accomplished via electrostatic control. An appliedpotential oxidizes the Fe⁰ to Fe⁺² with subsequent reduction of the AAOmembrane to insure charge balance. Once the Fe⁰ is oxidized to Fe⁺² astrong ionic bond between the CPA nanotube arrays and PAA pretreatedpaint will allow easy peel-off by separating, the two surfaces. Theoxidized Fe⁺² can be reduced back to Fe⁰ via a reverse potentialallowing the removed paint to essentially fall-off the appliqué (CPA).

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

What is claimed is:
 1. A molecular adhesive system for reversiblyjoining two surfaces, comprising: an anionic coating on a first of twosurfaces to be joined; a conductive polymer nanotube array on a secondof the two surfaces to be joined; wherein said conductive polymernanotube array is functionalized with metal nanoparticles; and anelectric potential applied across said two surfaces.
 2. The molecularadhesive system of claim 1 wherein said conductive polymer nanotubearray is functionalized with anionic groups configured to ionicallyattach said metal nanoparticles.
 3. The molecular adhesive system ofclaim 2 wherein: said anionic groups comprise CO2-, OH—,tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate,perchlorate, sulfate, chloride, bromide, nitrate, or functionalizedpolymers.
 4. The molecular adhesive system of claim 3 wherein: saidfunctionalized polymers comprise polyesters, polyethers, polyvinyl,polyalcohols, poly amides, polyoelfins, polyaromatics, polyacids,polysilanes, polysiloxanes, polyacrylic acid, polystyrene sulfonic acid,polymethacrylic acid, or poly phosphonic acids.
 5. The molecularadhesive system of claim 1 wherein: said anionic coating comprises aco-polymer of acrylic acid with triethyoxyvinylsilane (TEVS).
 6. Themolecular adhesive system of claim 1 wherein: said conductive polymernanotubes are comprised of aniline monomers, pyrrole monomers, thiophenemonomers or combinations thereof.
 7. The molecular adhesive system ofclaim 1 wherein: said metal nanoparticles comprise iron nanoparticles.